The thing to remember with intervals is that even though this list looks intimidating at first (because I included all the sharp notes, flat notes, and natural notes), as long as you count the "basic", or "natural" versions on your fingers, you can "adjust" the notes accordingly to make the sharp or flat versions.
What I mean by that is let's say you need a Perfect 5th above F# (F sharp). Instead of trying to count 7 half steps up from F#, just start on the "natural" version of F#, which is F.
Next, use your fingers, and starting with your first finger as F, count up 5 letters. You should end up on C (F, G, A, B, C). Easy.
Now, what was that note you started with? F#? Go back and raise that F you were counting with, up to F#. Now raise the C you landed on, up to an C#. And there you go. A Perfect 5th above F# is C#. You took both the F up to an F#, and the C up to C#, together.
But just a little disclaimer here, that works most of the time. Sometimes the layout of notes can be a pain and counting on your fingers doesn't quite work.
For example, if you were to do that same trick starting on B, you would count up 5 letters on your fingers and end up on F, which is great... except that F is not a Perfect 5th above B.
F# (F sharp) is actually the 7 half steps (or frets) apart that we need for a Perfect 5th, not F.
F is only 6 half steps apart. I know, it's lame, but you’ll get the hang of it. Just keep using the counting fingers trick, and watch out for that B and F group!